The invention concerns a spark plug for an internal combustion engine, comprising a shell, an insulator located in the shell and composed of a sintered ceramic material, as well as a center electrode heat-fused into an insulator, and a terminal stud that have an electrically conductive connection with each other and are located in the insulator. The invention further concerns a method for producing a spark plug.
Due to the different thermal expansions of platinum and ceramic material, spark plugs comprising a platinum center electrode heat-fused into an insulator have a slight gap between the ceramic and the center electrode that allows air or combustion gases to penetrate. For this reason, the components in the interior of the spark plug must be stable in the presence of these gases. It is therefore impossible, for example, to install a carbon-based burn-off resistor in the anterior region of the spark plug on the combustion chamber side, because the carbon would be oxidized at the high temperatures by the penetrating atmospheric oxygen. Additionally, contact pins must be made of materials that are stable in the presence of the penetrating gases. Contact pins having high thermal conductivity, e.g., those made of copper, can therefore not be used.
A spark plug is made known in WO 97/49153, about which it is proposed that the contact pin be replaced with an electrically conductive ceramic-metal mixture in order to prevent mechanical stresses, because the coefficients of thermal expansion would then be the same.
The object of the invention is to further develop a spark plug of the type described initially such that a gas-tight, reliable seal is ensured that can be produced cost-effectively. The object of the invention is further to create a method for producing such a spark plug.
With the spark plug of the present invention, the insulator and the cermet have the same or similar material properties, which ensures sealing. The fact that the material properties are the same yields advantages for production as well as operation: insulator and cermet can be easily sintered together, because they have the same shrinkage behavior. Since insulator and cermet also have the same thermal expansion, no gaps are produced as a result of different thermal expansions. As a result of the good seal that is achieved, materials can be used in the anterior region of the spark plug that are not sufficiently stable in the presence of air or combustion gases at the high temperatures occuring during operation, e.g., resistors having carbon as the conductive phase or contact pins made of copper and having good thermal conductivity. Only a relatively small quantity of metal is needed for the metallic phase of the cermet, which results in low costs for the spark plug.
According to a preferred exemplary embodiment of the invention, it is provided that the ceramic phase of the cermet is composed of Al2O3, and the metallic phase is composed of platinum or a platinum alloy. This cermet can be easily sintered together with the insulator, because it comprises the same sintering properties as the insulator.
According to a preferred exemplary embodiment of the invention, it is provided that a ceramic granulated material is used to produce the cermet, the granules of which are provided with a surface coating of a material having good electrical conductivity. Due to the difference in size between the granules of the granulated materialxe2x80x94which preferably have a diameter in the range between 90 xcexcm and 150 xcexcmxe2x80x94and the pulverized materialxe2x80x94the particles of which are less than 10 xcexcm in size, a ceramic micro-structure results after sintering having a network of thin metal tracks, e.g., made of platinum, that ensures sufficient electrical conductivity despite the small quantity of metal used. It is sufficient, for instance, for the metallic phase of the cermet to constitute a quantity between 10 and 15% by volume. The precious metal that is preferably used is therefore used sparingly.
Reference is made to the explanations hereinabove with regard for the advantages achieved with the method according to the invention.
According to a preferred exemplary embodiment of the method, it is provided that the granules of the ceramic granulated material are coated with the material having good electrical conductivity by stirring in a diluted suspension. In this fashion, the granules can be coated with the electrically conductive material, e.g., platinum, in cost-effective fashion, so that the electrically conductive network is produced in the interior of the cermet after the granulated material is sintered. As an alternative, the material having good electrical conductivity can also be applied to the granules of the granulated material using an organic binding agent, for instance, or it can be applied via vapour deposition or sputtering.